World champion triathlete Tim Don will be banned from competing in any future Olympic Games after becoming the latest major British athlete to fall foul of UK Sport's 'three hits' rule for drug testing.

Don, who won gold at the world championships in Lausanne last month, has been tested on nine separate occasions this year with no adverse findings, but was today handed a three-month suspension for missing three out-of-competition tests. An independent tribunal found that Don's blame in the affair was limited, and accepted that the mistake was unintentional, but current British Olympic Association byelaws mean all such offenders are automatically banned from all future Olympic teams.

He does not plan to appeal the decision, which represents the minimum punishment available for the offence. A joint statement from the tribunal and the British Triathlon Association (BTA) attributed his mistake to "a combination of forgetfulness and a lack of understanding of the new [testing] system". "The tribunal does not believe that Tim Don had any intent to take any prohibited substance," read the statement. "Nor that Don has ever attempted or had the intent of avoiding the rules relating to whereabouts testing."

Don himself has always been a strong advocate of drug-free sport throughout his career, and made no excuse for failing to adhere to UK Sport's national anti-doping policy. But he is not the first to have trouble with the new online 'Whereabouts' system, which requires athletes to regularly update an online database specifying where they will be available for testing for one hour a day at least five days a week.

If athletes are not at the stated location when UK Sport's doping control officer attends to take a sample, it is reported as a missed test to the respective governing body. Three missed tests over an 18-month period constitutes an anti-doping rule violation.

"I have never taken, or even considered taking, a performance-enhancing drug in my life," said Don in a statement. "And I am absolutely devastated to receive a suspension for contravening anti-doping regulations. I fully understand that it is my responsibility as a professional athlete to log my whereabouts on the UK Sport system and accept that it is due to my forgetfulness and lack of understanding of the online system following its launch last year that I have received this ban.

"However, I am a clean athlete who has been tested for drugs on nine separate occasions this year with no adverse findings, and the tribunal and British Triathlon acknowledge there is no way I was deliberately trying to miss a test. On two occasions I was travelling overseas to represent Britain in international competition and failed to amend my location details. On the third occasion I went to an athletics competition in Loughborough which ran late. I arrived back home later than intended and minutes after the one hour window that is given for testing had expired."

Whereabouts was introduced in July 2005, then reviewed and relaunched by UK Sport August 2006 in an effort to iron out some teething problems, but several athletes have continued to have trouble with it.

Commonwealth 400metres champion Christine Ohuruogu was suspended for a year in August for missing three tests, and may now feel aggrieved at the difference in length of their respective bans. Ohuruogu is currently appealing to the Court of Arbitration for Sport (CAS).